1. Field of the Invention
The present invention relates to an optical element suitable for a light source (light-emitting element), such as a light-emitting diode or the like, and a lighting device provided with such an optical element, and more specifically to an optical element suitable for improving the directivity of light (a ray of light) emitted from a light-emitting element and a lighting device provided with such an optical element.
2. Description of the Prior Art
In recent years, with an increase in the power of a semiconductor light-emitting element (solid-state light-emitting element), such as a light-emitting diode (LED) or the like, application of the solid-state light-emitting element to the illumination field has been rapidly advanced. The solid-state light-emitting element converts electric energy directly into light energy, having features of higher efficiency and less heat generation upon light emission than an incandescent lamp and a fluorescent lamp. However, the solid-state light-emitting element has a large angle of divergence of light emitted from the light emitting surface, thus leading to an enormous lens size, when used in a conventional optical system, in attempt to achieve condensation with low loss.
Thus, conventionally, various suggestions have been given with a view to condensing an emitted ray of light with low loss. Then, one of such suggestions relates to configuration of an optical element provided, around or above a light source such as a light emitting diode or the like having wide exit angle distribution, as means for adjusting the exit angle distribution of the light source.
For example, patent document 1 discloses an optical element that transmits, concentrically as viewed from the front, a ray of light emitted from a light source by a reflection surface of an inverted conic shape. This configuration permits providing a lens for a light-emitting element which is excellent in the visibility not only from close distance but also from great distance and which is also excellent in the viewing angle characteristics.
More specifically, the lens for a light-emitting element described above is formed into an inverted circular conic shape. When the light-emitting element is disposed at a light-emitting element fitting part that is formed at the bottom of the lens body, light emitted from this light-emitting element is totally reflected on the peripheral wall of the lens body and radiated forward. On the peripheral wall, a corner is circumferentially provided (formed in the circumferential direction) at one area or a plurality of areas. The corner described above is formed by changing the angle between the peripheral wall described above and the central axis of the lens body from the bottom described above to the front surface of the lens. Circumferentially providing such a corner at one area or a plurality of areas causes light from the light emitting element described above to be scattered forward at the aforementioned corner and radiated concentrically as viewed from the front of the lens.
Moreover, patent document 2 discloses an optical element that changes the direction of a ray of light emitted from a light source by using a refracting surface and a total reflection surface. This optical element controls the viewing angle of light generated by a light-emitting diode (LED).
More specifically, the configuration is as follows. A peripheral optical element as the optical element described above efficiently collects light from the LED and generates high light intensity within a limited, narrow, predetermined viewing angle with respect to the optic axis. The LED is centered in this peripheral optical element with the refracting surface directing light toward the peripheral optical element. The light is totally reflected inside by the reflection surface and then exits from the peripheral optical element through an exit surface. The viewing angle is determined in correspondence with the angles of the refracting surface and the reflection surface with respect to the optic axis.
Patent document 3 discloses an optical element that controls a ray of light emitted from a light source by using a bulk-type lens having a side surface as a total reflection surface. With this configuration, the optical element can be used for semiconductor light sources, such as an LED and the like, thus permitting convergence, divergence, collimation, and the like of all of light emitted by these light sources, and also permitting providing an extremely low-cost lens.
More specifically, the bulk lens described above is formed of an optical medium having (i) an apex, (ii) a bottom, (iii) an outer periphery, (iv) a concave part which is composed of a ceiling and an inner periphery, and which is formed from the bottom toward the apex. The concave part functions as a storage part for the light source or a light detector, the ceiling and the apex function as a lens surface, the inner periphery functions as a light entrance surface, the outer periphery functions as a total reflection surface, and the bottom functions as a reflection surface.
Patent document 4 discloses an optical element employing total reflection and refraction and having an exit surface divided into two regions including a flat surface portion and a lens portion. With this configuration, of light exiting radially from the LED chip position, the light at the outer periphery and the light at the center are transformed into parallel rays that travel forward by reflection on a paraboloid of revolution and by primary refraction, respectively. As a result, the light emitted is effectively used, thus achieving high efficiency.
More specifically, the center of the bottom cut surface of the parabolic rotary body serves as a LED chip fitting position. Moreover, formed on this cut surface is a concave portion of a substantially hemispheric shape with its center lying at the LED chip fitting position. On the other hand, on the center of the front surface of the parabolic rotary body, a ring-shaped groove is formed which has a convex portion so formed as to be convexed forward for primary refraction. With this groove, the outer front surface is formed into a planar shape with transparent resin.
Patent document 5 suggests an optical element which has a peripheral side surface of a spheroidal shape and which improves the directivity of a ray of light from a light emitting element by total reflection on the peripheral side surface. FIG. 24 is a sectional view showing the general configuration of the optical element described above. This optical element has a peripheral side surface 101 that is bowl-shaped, and has near the apex thereof a concave portion 102.
With this configuration, of rays of light emitted from the light-emitting element stored in the concave portion 102, those entering a side surface 102a of the concave portion 102 are primarily refracted at this position, totally reflected on the peripheral side surface 101, and then exits forward in parallel to the optical axis. On the other hand, those entering a top surface 102b of the concave portion 102 are refracted by a parabolic curved surface of the top surface 102b, and exits forward to the front surface as rays of light in parallel to the optical axis. This results in higher light use efficiency, thus permitting an increase in the luminance.
Patent document 6 suggests an optical element so formed as to have the side surface 102a of the concave portion 102 of FIG. 24 provided with curvature. This configuration, compared to the configuration of FIG. 24, seems to further improve the directivity of a ray of light from a light-emitting element.
Patent documents 1 to 6 are as follows:    [Patent document 1] JP-A-2005-174693    [Patent document 2] JP-A-H9-167515    [Patent document 3] JP-A-2002-228921    [Patent document 4] Japanese Utility Model Publication No. H 6-28725    [Patent document 5] Japanese Patent Publication No. H4-36588    [Patent document 6] Number of Published Japanese Translation of a PCT Application 2004-516684.
However, configuration as described in the patent documents above requires the following improvements.
Specifically, patent document 1 is targeted on a light source which is too small for the optical element. Thus, in order to provide the same effect even in a case of a light source with some width, such as the case with a surface light source, the size of this light source needs to be considered, thus resulting in the need for a very large optical element.
In patent document 2, which employs refraction and total reflection to change the direction of a ray of light from the light source, light emitted from the light source reaches the total reflection surface after passing through the refracting surface, which results in complicated configuration.
In patent document 3, which employs the bulk-type lens in order to increase rays of light emitted forward of the light source, the size of the lens is impractically very large when a surface light source with a large diameter is used as a light source.
In patent document 4, in which the exit side surface of the optical element has very complicated configuration, the degree of difficulty in manufacturing is great, thus disadvantageously leading to cost increase and lower efficiency.
With the configuration of the optical elements of patent document 5 and 6, it is assumed that, in order to refract rays of light entering the top surface 102b of the concave portion 102 shown in FIG. 24 to thereby improve the directivity thereof, the maximum diagonal length of the top surface 102b (maximum length perpendicular to the optical axis) needs to be at least twice the diagonal length of the light-emitting surface of the light-emitting element. Therefore, an increase in the area of the light-emitting surface of the light-emitting element requires an increase in the maximum diagonal length of the top surface 102b, thus resulting in an increase in the diameter of the optical element, which fails to provide a compact optical element.
In addition, the rays of light entering the side surface 102a of the concave portion 102 forms a larger angle with respect to the optical axis as a result of refraction on the side surface 102a. To subject the rays of light refracted on the side surface 102a to total reflection on the peripheral side surface 101 in this condition, the angle of incidence of the ray of light onto the peripheral side surface 101 needs to be larger than the total reflection angle. This requires a large inclination of the peripheral side surface 101 with respect to the optical axis, thus resulting in a large diameter of the optical element, which leads to failure to achieve compact configuration of the optical element.
In conclusion, the configuration of the optical elements suggested in patent documents 1 to 6 suffer, in efficiently condensing the light-emitting element (improving the directivity of a ray of light emitted from the light-emitting element), from: a problem of complicated form of the optical element, which leads to cost increase; and a problem of a large diameter of the optical element with respect to the size of the light emitting surface of the light-emitting element, which results in insufficient compactification.